In this paper, we present a physics-based model for two-dimensional electron gas (2DEG) sheet carrier density n(s) and various microwave characteristics such as transconductance, cut-off frequency (f(t)) of the proposed Spacer layer based AlxGa1-xN/AlN/GaN High Electron Mobility Transistors (HEMTs) is modeled by considering the quasi-triangular quantum well. To obtain charge density n(s), the variation of Fermi level with supply voltage and the formation of various energy sub-bands E-0, E-1 are considered. The obtained results are simple and easy to analyze the sheet carrier density, DC model and microwave frequency performance analysis for nanoscale Spacer layer based AlxGa1-xN/AlN/GaN HEMT power devices. The Spacer layer based AlGaN/AlN/GaN heterostructure HEMTs shows excellent promise as one of the candidates to substitute present AlGaN/GaN HEMTs for future high speed and high power applications. Derived model results for drain current, transconductance, current-gain cutoff frequency for different short and long gate length device are calibrated and verified with experimental data over a full range for gate and drain applied voltages and is useful for nanoscale and microwave analysis for circuit design. (C) 2013 Elsevier Ltd. All rights reserved.